In recent years, multimedia server systems have attracted a lot of attention. In a typical multimedia server system, a plurality of multimedia data including, for example, image data, voice data, and character data, are recorded in a storage device and, on a server-client system network, the server reads out the multimedia data from the storage device in response to the client's request and transmits the data to the client wherein the data is reproduced. In such a multimedia server system, when moving image data is transmitted, a sufficiently high transmission speed is required for continuous reproduction of the data. Hence, the storage device is desired to perform high-speed reading and writing of data. However, a high-speed storage device, such an a hard disk, is expensive. Since the multimedia server system needs a mass storage device, such as an RAID using a plurality of hard disks, the price of the system increases significantly. In order to spread the multimedia server system by reducing the price, in recent years, a multimedia server system employing, as a mass storage device, a library unit that deals with plural optical disks, which are cheeper than hard disks, has been developed.
A description is now given of a multimedia server system employing an optical disk library unit, and a method of reproducing multimedia data in the system.
FIG. 27 is a block diagram illustrating a prior art multimedia server system employing an optical disk library unit. In FIG. 27, reference numeral 500 designates an optical disk library unit containing a plurality of optical disks wherein a plurality of multimedia data are recorded. These multimedia data are data including image data and compressed according to a format based on general MPEG method. Reference numeral 400 designates a server, numeral 600 designates clients, and numeral 650 designates a network connecting the server 400 to the clients 600. The server 400 reads multimedia data from the optical disk library unit 500 in response to requests from the clients 600 and distributes the data to the clients 600. The clients 600 request the server 400 to read multimedia data, receive the multimedia data, and reproduce the multimedia data. Reference numeral 550 designates a connection between the optical disk library unit 500 and the server 400, on the basis of SCSI (Small Computer System Interface) standard or the like.
FIG. 28 is a schematic diagram illustrating the structure of the optical disk library unit 500. In FIG. 28, reference numeral 501 designates an optical disk in which multimedia data is recorded, and the recorded multimedia data is processed for a logical block as a unit. Reference numeral 502 designates a storage shelf comprising plural slots, each containing an optical disk. Reference numeral 503 designates an optical disk drive for reading multimedia data from the optical disk 501 or writing multimedia data in the optical disk 501. Reference numeral 504 designates a conveyer for conveying the optical disk 501 between the storage shelf 502 and the optical disk drive 503. Reference numeral 505 designates a controller for controlling the conveyer 504 and the optical disk drive 503 on the instruction of the server 400.
FIG. 29 is a block diagram illustrating the structure of the server 400. In FIG. 29, reference numeral 401 designates a library control means for controlling the optical disk library unit 500 connected to the server 400. Reference numeral 402 designates a control means for controlling reading and transmission of multimedia data in response to the requests of the clients. Reference numeral 403 designates a data transmitting and receiving means for exchanging data with the clients through the network 650. Reference numeral 404 designates a recording content storage means storing a title specifying multimedia data, an optical disk in which the multimedia data is recorded, and a position on the optical disk where the multimedia data is recorded.
FIG. 30 is a block diagram illustrating the structure of the client 600. In FIG. 30, reference numeral 601 designates a reproduction request accepting means for accepting a request for reproduction of multimedia data from a user. Reference numeral 602 designates a data reproducing means for reproducing multimedia data sent from the server 400 and displaying the data on a display unit 603. Reference numeral 604 designates a data transmitting and receiving means for exchanging data or requests with the server 400 through the network 650.
FIG. 31(a) shows the data structure of information stored in the recording content storage means 404 within the server 400. An shown in FIG. 31(a), the recording content storage means 404 stores a title of each multimedia data recorded in an optical disk, information for identifying the optical disk in which the multimedia data having the title is recorded (hereinafter referred to as an identification number of the optical disk), a logical block address showing the position on the optical disk where the multimedia data is recorded, and a slot number showing the position of the optical disk in the storage shelf 502.
FIG. 31(b) shows examples of the stored data. In this case, multimedia data with a title "multi05" is recorded in an optical disk "disk#3" in a position from a logical block 0 to a logical block 8911, and this optical disk is contained in a slot #11 in the library unit.
FIG. 32 is a flowchart for explaining a method for reproducing multimedia data using the prior art multimedia server system.
Initially, in step 1, a user requests reproduction of multimedia data to the reproduction request accepting means 601 in the client 600 by designating the title of the multimedia data, for example, "multi05".
In step 2, the request for reproduction of "multi05" is transmitted from the reproduction request accepting means 601 to the data transmitting and receiving means 604. Further, the request is transmitted through the network 650 to the server 400. In the server 400, the request from the client 600 is received by the data transmitting and receiving means 403, and the control means 402 checks the data in the recording content storage means 404 to find the optical disk having the multimedia data requested by the client 600. More specifically, since the recording content storage means 404 stores the data shown in FIG. 31(b), the control means 402 finds the identification data of the optical disk, the storage position of the optical disk, and the position of the requested data on the optical disk, on the basis of the title "multi05" of the requested multimedia data. In this case, the control means 402 finds that the desired multimedia data "multi05" is recorded in a range from logical block address 0 to logical block address 8911 on the optical disk "disk#3" contained in the slot #11 in the storage shelf.
In step 3, the control means 402 confirms, through the library control means 401, whether the optical disk "disk#3" is mounted in the optical disk drive 503 in the optical disk library unit 500. When the optical disk is not mounted in the drive 503, in step 4, the control means 402 instructs the library control means 401 to convey the optical disk "disk#3" from the slot #11 of the storage shelf 502 to the optical disk drive 503. When the optical disk "disk#3" is already mounted in the optical disk drive 503, the control means 401 proceeds to step 5.
In step 4, the library control means 401 controls the optical disk library unit 500 so that the optical disk "disk#3" in the library unit 500 is conveyed from the storage shelf 502 to the optical disk drive 503. The controller 505 in the optical disk library unit 500 controls the conveyer 504 to take the optical disk "disk#3" from the slot #11 of the storage shelf 502 and insert the optical disk in the optical disk drive 503.
After confirming whether the optical disk "disk#3" is mounted in the drive 503, the control means 402 proceeds to step 5. In step 5, the control means 402 instructs the optical disk library unit 500 to read multimedia data, through the library control means 401, on the basis of the logical block address obtained from the recording content storage means 404. In the optical disk library unit 500, the controller 505 instructs the optical disk drive 503 to read multimedia data from the designated logical block address 0. Then, the server 400 receives the multimedia data read from the optical disk, and the data transmitting and receiving means 403 transmits the multimedia data, through the network 650, to the client 600, followed by step 6.
In step 6, the multimedia data is received by the data transmitting and receiving means 604 in the client 600 and reproduced by the data reproducing means 602.
Next, in step 7, it is judged whether all the data recorded in the logical block addresses 0-8911 are reproduced. When the reproduction Is not completed yet, the above-mentioned reading, transmission, and reproduction of the multimedia data are repeated.
In the method for reproducing multimedia data using the prior art network system, when the multimedia data requested by the client is recorded in a single optical disk, this data can be continuously output from the server and continuously reproduced in the client. However, if the requested multimedia data is recorded over plural optical disks, reading and transmission of this data are not performed when the optical disks are exchanged. Consequently, reproduction of the data is unfavorably interrupted due to the exchange of the optical disks.
As described above, the optical disk library unit can store a mass of multimedia data at a relatively low cost. However, in many cases, multimedia data, such as image and voice, is recorded over plural optical disks. Hence, such an interruption in reproduction of multimedia data is an undesirable matter for the multimedia server system.
A description is now given of a case where plural clients request for reproduction of multimedia data from a single optical disk at the same time. It is assumed that two lines of MPEG1 data respectively recorded on an outer circumference and an inner circumference of an optical disk are reproduced using an optical disk drive having the following performances: maximum seek time of about 750 msec; maximum rotation waiting time of about 30 ms; effective transmission rate in a range from 520 Kbytes/sec (inner circumference) to 1150 KBytes/sec (outer circumference).
In order to reproduce the two data lines continuously, data of 187 KBytes/sec must be read out from both the inner circumference and the outer circumference. The maximum time required for the reading is calculated as follows:
______________________________________ data reading time = rotation waiting time * 2 + inner circumference data transmission time + outer circumference data transmission time + seeking time = 30 * 2 + 187/520*1024 + 187/1150*1024 + 750 (ms) = 1344 (ms) = 1.3 (s) ______________________________________
Since the data reading time exceeds one second, it is not possible to read out data at a sufficient speed for continuous reproduction. That is, in this case, reproduction of multimedia data is interrupted. As mentioned above, the possibility of reproduction in response to plural requests from plural clients to a single recording medium depends on the recording medium and the performance of the device processing the recording medium, i.e., it depends on the speed of reading or writing data on the recording medium. Therefore, when plural clients request, at the same time, reproduction of multimedia data recorded in a high-speed storage device, such as a hard disk (magnetic disk), the storage device can deal with the requests. However, when an optical disk drive having a relatively low data reading speed is employed, if the clients' requests for reproduction of data concentrate on a single optical disk, the reproduction is unfavorably interrupted as described above.